Organic molecule based field effect transistors (FETs) have been a subject of intense research efforts over the last decade. The popularity regarding these FETs is primarily because these devices are generally fabricated using lower cost capital equipment, and lower temperatures as compared to traditional silicon based transistors. However, organic FETs are not necessarily useful in high performance computing or very high speed applications (in the near term), but are expected rather to replace silicon in high volume applications where very small size/speed is not required, but cost is an issue. Examples include the switching transistor drivers for each pixel in the back planes of LCDs and electroluminescent displays and in radio frequency identification tags (RFIDs). Organic FETs thus provide the promise of cheap, transparent, flexible electronics. However, organic FETs are plagued by a low mobility of charge carriers that limit performance. The low mobility requires a larger source-drain voltage to drive the currents needed, which translates into higher power consumption.
The performance of a FET is generally characterized by the mobility (μ) of the carriers in the active channel, the current ratio between the on/off states and the subthreshold slope, which specifies the gate voltage needed to switch between the on/off states. High values of μ(>about 0.1 cm2/Vs) and on/off ratios (>106) and low subthreshold slopes (<500 mV/decade) are desirable for practical application of FETs in circuits. The mobility of organic semiconductors is dependent on the degree to which the charge-accepting orbitals of neighboring molecules overlap. In contrast to inorganic semiconductors, such as Si where charges move within a three-dimensional covalent network, organic semiconductors rely upon weak van der Waals interactions between discrete molecular constituents and charge transport relies on intermolecular hopping. The mobility in organic semiconductors increases with improved crystallinity, however even perfectly ordered organic semiconductor crystals have much lower mobilities than covalently bonded crystalline semiconductors. One way to overcome the limitation imposed by this intrinsically lower mobility is to make the distance the charges must travel small, that is, to make the channel length between the source and drain terminals short.
The term “organic semiconductors” is used to describe molecular organic solids in the form of a bulk layer or Elm, which possess the ability of transporting charge. The electrical conductivity of these materials lies between that of metals and insulators, spanning a broad range of 10−9 to 103/ohm-cm. Depending on the specific organic molecules, the intentional or incidental doping and/or the contact electrode materials used, that conductivity can occur via electrons near the bottom of the conduction band (n-type conductivity) or holes near the top of a valence band (p-type conductivity). Pentacene is an exemplary organic semiconductor that is commercially available or can be readily synthesized in the laboratory. Most FETs fabricated with pentacenes exhibit p-type conductivity.
A cross-sectional schematic of a standard organic molecule p-type FET (PFET) 100 configuration is shown in FIG. 1. FET 100 includes a p-doped back gate 101, dielectric layer 102, drain metal electrode 105, an organic semiconductor layer 103, such as a pentacene layer, and source metal electrode 104. In a typical fabrication of such a device the source and drain electrodes are formed on the gate dielectric layer followed by deposition of the pentacene layer (so called “bottom contacts”), however, other constructs are also common. For example, the organic molecule layer can be deposited onto the gate dielectric followed by formation of the source/drain electrodes (so called “top contacts”). There are also top gate constructs in which the dielectric layer may be deposited on top of the organic semiconductor layer, followed by the deposition of a gate electrode. As known to those skilled in the art, because of the symmetry between the source and drain electrodes, the source and drain can interchange roles for operation of the transistor with no change in the device performance.
Referring again to FIG. 1, the cross-sectional view provided is a slice through the device, perpendicular to the long axis of the electrodes, which extends for a length greater than their width into the page. The channel length of the device is the distance between the source 104 and drain 105 electrodes indicated in FIG. 1 by CL. In operation a small fixed voltage is applied between the source 104 and drain 105 electrodes. Modulation of the gate voltage (Vg) controls the carrier concentration in the channel region and as a result the source-drain electrode current. For a p-type silicon back gate 101 and large work function metal source-drain electrodes (e.g. Pd) the positive potential on the source 104 and drain 105 electrodes, relative to the negative gate 101, draws electrons out of the normally filled, highest occupied molecular orbitals (HOMOs) of the pentacene or other organic semiconducting active layer 103 resulting in an increase of the hole carriers shown as + + + + charges in FIG. 1. The resulting increased hole carrier density in the pentacene or other organic semiconducting layer results in the increased source drain current that turns the transistor on. A positive gate (VG>0) potential fills the HOMOs, minimizing the hole carrier density, turning the transistor off.
Short channel lengths improve organic molecule based transistor performance. However, fabricating channel lengths of the order of 100 nm or less for the configuration shown in FIG. 1 is technologically challenging, particularly for mass production.